Background and Aims: Long QT syndrome (LQTS) ablation is currently limited to high-risk or refractory cases, targeting the elimination of triggers such as ventricular premature contraction (VPC). However, the characteristics of the underlying substrate and the role of radiofrequency ablation have not been extensively discussed. Methods and Results: Eight symptomatic patients with LQTS (all women; aged 36 ± 10.2 years) underwent 3-D endocardial and epicardial mapping of triggers and/or substrates of ventricular arrhythmias. Genetic testing revealed P1093A mutations in the KCNH2 gene, H558R mutations in the SCN5A gene, and K28E mutations in the KCNH2 gene in three patients, respectively. A total of 13 VPC morphologies were identified, including five right ventricular outflow tract (RVOT) VPCs, one basal lateral RV, two from inferoapical RV, one from the lateral to annulus tricuspid, three from basal lateral LV and one from epicardial site of the lateral mitral annulus. Epicardial voltage mapping revealed moderate to extensive areas of scar and low voltage in all patients. The localized abnormal ventricular activities (LAVAs) were recorded in all patients and were successfully ablated. The corrected QT interval (QTc) shortened after the epicardial LAVA elimination (594.9 ± 85.98 ms vs. 490 ± 67.49 ms, p = 0.001). During a mean follow-up period of 288 ± 147.4 d (range 170–492 d), two patients experienced ventricular tachycardia (VT) /ventricular fibrillation (VF) recurrence. Conclusion: Regional and limited epicardial LAVAs could be identified in patients with LQTS. Radiofrequency ablation of VPC triggers and epicardial LAVAs had a modest effect on preventing future VT/VF recurrences. © 2025 The Author(s)